1. Field of the Invention
The present invention is in the field of forming metallic conductors and relates more particularly to a method of depositing encapsulated conductive paths on a substrate.
2. Prior Art
In the manufacture of electronic components such as resistors, capacitors and the like it is often necessary to effect electrical connection to or between one or more terminals exposed at a surface of the component, the terminals being in electrical connection with functional elements of the component buried within the body of the device. By way of example, and without limitation, a capacitor may be comprised of a ceramic monolith having a number of buried electrodes. The electrodes may include spaced tabs extending to a surface of the monolith. With a structure as described, the capacitance of the device may be adjusted by electrically connecting all or less than all of the exposed tabs to achieve a desired value.
It is often necessary, for a given electrical component, to form a multiplicity of such conductive paths on a given surface of the component, for instance to form respective conductive paths linking the electrodes of opposite polarity of a capacitor, or for forming discreet capacitors within a single monolith.
The formation of conductive paths on the surface of electronic components is complicated by the fact that it is often necessary that the conductive paths be comprised of a series of superposed layers. By way of example, due to such factors as migration and chemical inter-action it is necessary first to apply a metallic layer which is compatible with the material of the exposed tabs. Such material may not provide an ideal conductor or may be expensive and hence its use in limited quantities is called for. In order to render the path suitably conductive, it may thereafter be necessary to apply a second, highly conductive metal which will not adversely react with the first applied layer but which could not be directly contacted with the tabs. Thereafter, in order to shield the highly conductive layer from effects such as oxidation, it may be desired to overcoat the highly conductive layer with a still further layer compatible with the other layers and resistant to oxidation.
The formation of metallic conductive paths by such metal deposition steps as plating, sputtering, and ion deposition, is well known. Heretofore, the formation of conductive paths having superposed layers has involved the use of multiple masks which must be sequentially applied in accordance with the width of the desired conductive path. If, for instance, it is desired to apply a conductor comprised of an aluminum layer encapsulated within upper and lower passivating tungsten layers, the procedure heretofore practiced was to mask the electronic component with a first wide mask, apply a tungsten layer, remove the first mask and position a narrower mask in registry with the deposited tungsten layer, apply a narrower strip of aluminum within the opening of the second applied mask, remove the second mask and re-mask the device with a third mask of greater width than the mask used to apply aluminum, and deposit the covering tungsten layer through the third and wider mask.
As will be apparent to those skilled in the art, steps of masking and re-masking involve significant problems of accuracy in mask location, considering particularly that the electronic components may, in their largest dimension, be only fractions of an inch in size.
Patents relating to metal deposition methods of various sorts but which fail to provide a solution to the above referenced problem include:
(A) U.S. Pat. No. 4,536,942. This patent relates to forming a T-shaped electrode by sequential angular and perpendicular metal deposition steps.
(B) U.S. Pat. No. 4,580,331 teaches angular evaporation of applied photo-resist to achieve a desired patterning effect.
(C) U.S. Pat. No. 4,024,041 discloses the use of a mask having a thermally stable elastic membrane on its under surface so as to prevent the spread of metal deposited through an aperture in the mask.
(D) U.S. Pat. No. 4,171,234 discloses a method of forming a crystal having a desired epitaxial crystal growth factor through the use of molecular beams angularly oriented with respect to the target crystal.
(E) U.S. Pat. No. 4,330,932 discloses a process for forming a semi-conductor device which includes depositing a thin film on a substrate through first mask, implanting the depositing film with a dopant, thereafter positioning a second mask over the noted films and depositing an electrical contact through the second mask.
(F) U.S. Pat. No. 4,410,401 discloses a material deposition step wherein the depositing device includes a shutter having an object obscuring portion, the device being adapted to remove contaminants by collecting the same in a trap formed on the object obscuring portion of the shutter.
(G) U.S. Pat. No. 4,060,427 discloses a method of forming an integrated circuit through a combination of ion implantation and diffusion steps. The process includes depositing first and second mask forming layers on the substrate, cutting a narrow aperture in the upper mask forming surface, forming a wider aperture in the lower mask forming surface to define an undercut, forming a first ion implantation area through the mask and thereafter forming by diffusion a wider layer extending to the limits of the undercut portion.